Computer Aided Design (CAD) systems are used to design and simulate virtual models of electrical, electronic or mechanical devices prior to producing actual physical devices. CAD systems are interactive software tools that run on a digital computer with a graphical display device. In particular, micro-fabricated devices such as electronic integrated circuits (ICs) and Micro Electro-Mechanical Systems (MEMS) can be designed and simulated using CAD systems prior to beginning the costly and time-consuming process of fabricating actual physical devices. The micro-fabrication process (or “process”) for MEMS and IC devices involves depositing multiple layers of material on a silicon wafer and optionally etching each layer with a patterned mask to define the device shape. The functionality of both ICs and MEMS devices depends strongly on this process.
MEMS are micro or nano-scale devices typically fabricated in a similar fashion as integrated circuits (ICs) to exploit the miniaturization, integration, and batch processing attainable with semiconductor manufacturing processes. Unlike ICs which consist solely of electrical components, MEMS devices combine components from multiple physical domains and can contain, for instance, electrical, mechanical, and fluidic components. MEMS devices include, for instance, micro-electromechanical sensors and actuators such as gyroscopes, accelerometers, and pressure sensors, micro-fluidic devices such as ink jet heads, Radio-Frequency (RF) devices such as switches, resonators and passives, and optical devices such as micro-mirrors.
The behavior of both MEMS and IC devices can be modeled at the system level, that is, as an interconnected network of simpler components. Each component has an underlying mathematical description, or behavioral model, which is referred to herein as a component model. Typically, these component models are parameterized, i.e. they take as input a few parameters such as width and height, so that the same mathematical model can be used for different versions of the same type of component. For example, a single component model may be used to generate models having different dimensions. A system-level simulator numerically computes, or simulates, the collective behavior of the network of component models.
Two commonly used methods of describing a system-level simulation are circuit simulation and signal-flow simulation. A system-level design is captured graphically in a circuit schematic or in a signal-flow diagram, and then its behavior is simulated by, respectively, a circuit simulator or a signal-flow simulator. Traditionally, circuit simulation has been used for electronic circuit design while signal-flow simulation has been used for control system and signal processing design. Currently, both types of system-level simulation are used to simulate not only ICs, but multi-physics devices such as MEMS.
Since MEMS devices are fabricated in a similar fashion as ICs and can also be simulated by system-level methods such as circuit simulation, CAD systems for IC design can be applied to MEMS design, at least in principle. In particular, IC schematic capture tools and circuit simulators can be applied to MEMS design when supplied with a library of MEMS component models.
Unfortunately, while MEMS and IC design share aspects related to manufacturing, they differ in the impact manufacturing has on their design flows. In particular, the micro-fabrication processes for IC devices are standardized. IC components are fixed within a fabrication process, while MEMS components are not. For instance, a transistor (an IC component) is created out of specific layers deposited on the silicon substrate during the fabrication process and these layers cannot be changed by the IC designer, but a mechanical beam component that is part of a MEMS design can be placed on any layer and that layer is a design choice. Conventional IC design tools do not offer the flexibility to change the location of a component within the various layers deposited during the fabrication process. Thus the details of the chosen fabrication process of an IC are fixed from the beginning and do not change from one design iteration to the next. In comparison, the fabrication processes of MEMS devices are not standardized. It is often necessary to tailor the fabrication process to a particular MEMS device in order to achieve the design goals for the device. Thus the fabrication process is an important “free parameter” in MEMS designs that will likely need to be changed as the design of a MEMS device progresses. The flexibility to change the description of the fabrication process is missing from IC design environments.
An additional problem with the use of conventional IC/MEMS design environments is that the mathematical models of electrical IC components can not be parameterized in terms of the process parameters since IC processes do not vary as part of the design. In MEMS design, the parameters of the process description can be varied as part of the design and the mathematical models must be parameterized with respect to the process parameters. The user must specify all of these process parameters in an IC schematic editor. Since there may be hundreds of such parameters, specifying this data and changing it throughout the design process is time-consuming and subject to error.